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Methods for exploration of thermal waters and a geological-commercial assessment of their deposits
Geethermics (I97o) - SPECtXLISSUE 2 U. N. Symposiumon the Developmentand Utilization of Geothermal Resources, Pisa I97o. Vol. 2, Part 2
Methods for Exploration of Thermal Waters and a GeologicalCommercial Assessment of Their Deposits S. S. BONDARENKO*, B, F. MAVR1TSKY * AND L. F. POLUBOTKO *
ABSTRACT The concepts of the regularities of thermal water occurrence, the conditions of their occurrence and circulation and hydrogeothermal indices of thermal water deposits are the basis of the methods of thermal water exploration. According to the conditions of occurrence and circulation all thermal waters are divided into two types: stratal and fissure-veined waters. Stratal thermal waters occur in platform regions, foredeeps and intermontane areas, and extend to m o s t large areas (tens and hundreds of thousands of square kilometers). The fissure-veined thermal waters occur ,in folded areas and have a local nature of outlet and extend to small areas (from portions to units of square kilometers). The methods of exploration and determination of the exploitation resources of thermal water deposits of stratal and fissure veined type are different. Based on the analysis of the available geological, hydrogeological and geothermal data on the basins in which stratal thermal water deposits are found, the principal aquifers and complexes with thermal water of usable quality for heat supply are determined and distinguished. Exploration of these aquifers, and complexes distinguished, is conducted for determination of calculating parameters required for calculation of exploitation resources. These resources are determined by means of hydrodynamic computation, taking into account the boundary conditions (infinite bed, semi.finite bed, bed-strip, etc.). Exploration of the fissure-veined thermal water deposits is carried out in previously established thermal anomalies using the ground geophysical methods and the complex hydrogeothermal survey at a large scale, by means of which the structure and sizes of a deposit, composition and temperature of thermal waters (superheated waters) and the approximate possible heat quantity of a deposit are determined. The preliminary data obtained are made more precise by drilling of exploration wells, and the deposit resources are estimated by the method of pumping tests of thermal waters fronf a group of wells. The hydraulic method is the principal one for determining exploitation resources of the fissure-veined thermal waters. In conducting exploration works at the thermal water deposits (and superheated waters) the main technical-economical indices are defined. The question of industrial estimation of the thermal water deposits arises before the hydrogeologists at all stages of their exploration, from the stage of prospecting works to their exploitation for industrial purposes. The economic significance of thermal water deposits, and economic efficiency in their exploitation are established on the basis of geological-economic estimation. The above methods enable the reseacher to plan the succession of development of thermal water deposits. The application of methods for reconnoitring and exploration of minerals depends on the peculiarities of their origin, regularities of distribution and conditions of occurrence. Elucidation of the geological situation * Union Research Institute of Hydrogeology and Engineering Geology (VSEGINGEO), Moscow, USSR.
and hydrothermal conditions which influence the formation of thermal water deposits is a prerequisite for selecting and substantiating the methods and direction of research and exploration of thermal waters and steam. Investigations, which have been carried out by various geological organizations of the USSR Ministry of Geology and the USSR Academy of Sciences, have made it possible to establish the general regularities of thermal water and steam occurrence within the confines of the Soviet Union, and reveal prospective regions where these waters can be utilized for electric power generation, introduction of district heating plants, balneology, etc. (GOLDBERG, YAZRIN 1965; MAVRITSKY,ANTONENKO 1967; PLOTNIKOV 1968). According to the conditions of occurrence and character of circulation, the thermal waters of the prospective regions are subdivided into stratal and fissure-vein. The former occur in vast areas within the confines of Epihercynic platforms, foredeeps and intermontane depressions composed of the Mesozoic and Cenozoic sedimentary deposits, and are predominantly united to form large head-water systems. The latter are characterized by local distribution and are generally confined to the places of intersection of large fractured zones with deep erosion cuttings. These deposits are connected with head-water systems in the discharge areas of the fissuretype. An approximate subdivision of the deposits of thermal waters and steam into types is given in Table 1. The deposits of thermal waters in platform areas may be rather considerable, which calls for the singling out of exploitable sites (within the confines of such deposits) where it is expedient to utilize them, An accurate determination of the boundaries of thermal water deposits in a number of regions is hampered by the gradual change of their parameters (temperature, salt content and chemical composition of subterranean waters, thickness and depth of water-bearing rocks, etc.). In such cases (for instance, in the West-Siberian artesian basin) the boundaries of deposits are only roughly outlined. The scope of hydrogeological prospecting is mainly dependent upon the amount of knowledge concerning the particular deposit accumulated during the period of 1175
TABLE 1. Type of deposits
'~ "~
-
-
Classi/ication o/ the deposits o/ thermal waters.
Class of deposits (according Kinds of deposits to water temperature) (accordingto salt content)
Character of flow
Hydrodynamic features
1. Low-thermal (from 200 to 50 °C)
I. Fresh (up to 1 g/l)
1. Flowing, unsaturated with gas
1. Continuous stratum
2. Thermal (from 500 to 100 °C)
2. Brackish (1-10 g/l)
2. Flowing, saturated with gas
2. Semirestricted stratum
3. Highly thermal (more than 100 °C)
3. Saline (10-35 g/l)
3. Eruption of steam and water mixture
3. Stratum-band 4. Stratum-quadrant
1. Low-thermal (from 20o to 50 °C)
1. Fresh (up to 1 g/l)
1. Flowing, unsaturated with gas
2. Thermal (from 50o to 100 °C)
2. Brackish (!-10 g/l)
2. Flowing, saturated with gas
3. Highly thermal (more than I00 oC)
3. Saline (10-35 g/l)
3. Eruption of steam and water mixture
LL
time preceding direct investigations. In order to make reconnoitring and exploration more efficient, they should be conducted by stages and in strict succession. The aim of the first stage of exploration is, first of all, to reveal a deposit of thermal waters and single out a site (or several sites) prospective from the viewpoint of extracting and utilizing these waters in accordance with hydrogeological, technological and economic factors. The search for thermal waters can be subdivided into two stages. The first stage involves the study of the geological structure of the territory; the general regularities of distribution of subterranean thermal waters; depths and lithology of aquifers; chemical and gas composition of subterranean waters. This study should be based on the results of the preceding explorations, hydrogeological as well as deep test drilling and boring for oil. At this stage the predicted resources of thermal waters of the deposit are assessed and the hydrogeological conditions and possible resources of thermal waters on the revealed prospective sites are compared, with the object of developing them immediately. The second stage, exploration boring, involves investigation of the geological profile of rocks composing the deposit, determination of depths and thicknesses of aquifers, collector properties of water-bearing rocks, chemical and gas composition of thermal waters, heads of the subterranean waters and a study of the tapped aquifers by pumping. The exploration work is also aimed at specifying the depths and thicknesses of the water-bearing complexes and aquifers, estimating their water resources, and investigating the chemical composition and regime of subterranean waters. 1176
1. With one thermal water producing tectonic fissure (zone) 2. Block-like structure (a number of tectonic, mutually intersecting fissures)
The calculated and specific yields of wells, depths of dynamic levels (decreases in heads) of subterranean waters in the wells over the period of exploitation, optimum exploitational regime of the wells and deposit (or part of it) must be determined based on an analysis of the investigation results. Study of the standard demands on the deposit of thermal waters and the estimation of their exploitable resources are the purpose and most significant task of reconnoitring and prospecting. Experience shows that inadequacy of the revealed exploitable resources considerably prolongs the period of development and increases expenditures on preparation of the deposit for commercial use. During the period of prospecting it is obligatory to define the size of the exploitational site, as it determines the volume of exploitable suberranean water resources and the number of exploitational wells. Analysis of the hydrodynamic situations of deposit exploitation has proved the necessity of limiting the areas of exploitable sites under platform conditions, to ensure the most efficient exploitation of subterranean water resources. An unjustified increase in the area of the site under exploration will bring about an appreciable increase in the cost of prospecting work, and will result in an insignificant yield of the exploitable resources of subterranean thermal waters, which is stipulated by the peculiarities of distribution and regime of the subterranean thermal waters within the confines of large geological structures. An elastic-pressing filtration regime of subterranean waters is characteristic of the majority of thermal water deposits in artesian basins. The collector properties of the deeply seated aquifers are usually characterized by small values of hydraulic conductivity (generally less than 1 m/day) and water transmissibility (from several
dozen to 1()0-150 and seldom more m2/day). T h e initial yields of i n d i v i d u a l wells (especially u n d e r excess of head) can be considerable. They will, however, decrease w i t h time due to the d e v e l o p m e n t of a depression cone and, mainly, owing to the interaction of wells during their exploitation. The discharges of water intakes usually fluctuate from 50 to 200-300 1/sec ( T a b l e 2). The above considerations necessitate a c o m p l e x assessment of thermal w a t e r deposits taking into account the technical and economic indices relating to extraction and utilization of thermal waters in concrete conditions a n d regions. Assessment of the exploitable resources of thermal waters on the site of the w a t e r intake u n d e r design in p l a t f o r m conditions includes: •
A wealth of special publications deals w i t h the d e t e r m i n a t i o n of p a r a m e t e r s of water-bearing rocks. Therefore, there is no need to consider this subject in detail.
)
1)- Evaluation of the calculated hydrogeological parameters of the water-bearing rocks inside and outside the water intake site according to the data provided by boring and exploratory hydrogeological testing of wells. 2) - Schematization of the analysed hydrogeological conditions in the zone of possible influence of the water intake and elaboration of the basic calculation of the hydrodynamic scheme. 3) - Expression in figures of the standard demands on thermal waters and conditions of their exploitation by accomplishing consecutive hydrodynamic and technico-economic calculations based on the variational method. 4) - Calculation of the total yield of the water intake (taking into account the standard demands) with respect to the most rational system (number and distribution) of the exploitational wells. This yield is regarded as the exploitable reserves of subterranean thermal waters. 5) - Classification of the exploitable resources according to the availability of investigational data for the particular site, trustworthiness and degree of reliability of the initial parameters.
T h e accuracy of p a r a m e t e r s of deep-seated aquifers is affected by a n u m b e r of factors, including: a ) - changes of teraperature in the borehole during different periods of exploratory hydrogeological investigations; b) - gas saturation of subterranean waters; c ) - hydrodynamic imperfection of a well; d) - resistance to the water movement in tubes on their way from the aquifer to the mouth of the well. These mainly decrease the dynamic levels of excessive pressure at the m o u t h of the flowing wells. U n d e r e s t i m a t i o n of the above factors m a y entail grave errors in determining p a r a m e t e r s of water-bearing rocks and exploitable resources of s u b t e r r a n e a n waters. The t e m p e r a t u r e and, consequently, w a t e r volume weight changes in a borehole are d e p e n d e n t on both the value of geothermal gradient and the t e m p e r a t u r e on the land surface. The difference is the greatest between the temperatures of w a t e r in the aquifer and at the static level (or at the mouth) when p u m p i n g has not been effected. This is the case w h e n static levels or excess pressures are measured in the test wells or static and dynamic levels (pressures) in the observation ones. In the course of p u m p i n g (test release) the w a t e r temperature at the m o u t h of the well d e p e n d s on the w a t e r t e m p e r a t u r e in the aquifer and w a t e r velocity in the borehole, i.e., on the yield. Therefore, the average volume weight of water in a borehole w h e n measuring
T^aL~ 2. - - Characteristics o~ thermal water deposits. Deposits
Kolpashevsk BarabainskKupinsk Tashkent
Makhasehkala
The main thermal water-bearing complexes
Approxi. mate depths of wells, m
Salt content, g/1
Neocomian and Apt-Cenomanian -- ~ --
1100-2200
1-5
1100-2000
1-15
Cenomanian
1500-2200
Plains o/ Uzbekistan 1-3 up to 150
Kizlyar
Chokrakkaragansk -- ~ --
1000-3000 2200-3000
Saki-Evpatoria
Neocomian
1000-1700
Excessive heads, m
t ° of water Piezoconat the point ductivity, of out-flowing mZ/day
Average wa- Calculated ter trans- yields of a missibility standard mZ/day water intake, 1/see (1)
West Siberia 20-50
35-75
105
190
150-250
10-30
35-65
105
200
130-250
50-75
5.105
Eastern Predkavkazje 2-20 20-50 10-20
100
The Crimean plains 5-10 up to 120
30
up to 60-70
45-90
5.105
250
300-400
80-100
105
150
up to 300
40
50-70
50-60
5.105
(~) A water intake with an area of 25 km z and 5 well.
The calculated time of exploitation: 104 days; level decline: 100 m below the earth surface. 1177
the static level will exceed that when measuring the dynamic levels in the process of pumping. In the first approximation, a level decline in the aquifer will be equal to:
T^aLE 4. -- Examples o~ the possible errors made while assessing coe[[icients o[ permeability due to changes in tem. perature.
Wells a) in a producing well Sd --
-~o.-
At,!
- - H Y'' ~
yat + 1 0 AP,_____~x
2ys
7,
(1)
y,
b) in a non-producing well Sd
- -
y,t--Ydt
+S
2Y,
7dl+Ys
-- head of subterranean waters in the well (in the case of a flowing well, the depth to the confining bed of the aquifer): --measured level decline, in m.
Table 3 contains the calculated values of errors AS (when determining a level decline Sd) dependent on the temperature at the well mouth before pumping out (t%0 and in the process of flowing (t°g) for the water with a salt content of 20-25 g/1. TABLE 3.
The calculated values o/ errors A S depending on the temperature ]'actor. -
-
Temperature Temperature at the sur- at the surface (t°.,) face during pumping out (t°,) I0
20
50 55 60 65 70
50 55 60 65 70
y.,
y,
H (m) 1000 [ 1600
0.76 0.28 0.54
4
1.48
0.18
Average
1.22
0.44
ration for the given volume and composition of the saturating gas, the gas starts escaping from the water. The specific weight of the formed mixture of gas and water is smaller than that of water without gas. Therefore, the fixed level decline (pressure) will not fully correspond to the decrease in the formational pressure. For the same conditions of the Tobol region, Tjumen area, an additional water level decline (A S) at a salt content of 18 g/l, gas factor 1.0, pressure of the saturating gas 68 atm, is 40.7 m (at P excessive = 1 atm), 32.2 m (at P,x = 2 atm), etc., i,e., the value of A S is comparatively high even at small values of the excessive pressure and gas factor. Partial penetration of wells (according to the degree and character of aquifer tapping) is responsible for additional resistance and head losses on the water movement path directly in the face zone and in the screen; this considerably decreases their water capacity. That is why the formulae used for determining the parameters contain the radius of an equivalent, fullypenetrating well instead of the actual one:
A S per A S per 1000 m 1600 m 1.0144
1.0125
1.0016 0.9993 0.9967 0.9948 0.9928
6.4 7.5 8.8 9.8 10.8
10.2 12.1 14.2 15.7 17.3
1.0016 0.9993 0.9967 0.9948 0.9928
5.5 6;6 7.9 8.9 9.8
8.7 10.5 12.6 14.2 15.8
By way of an example to illustrate the possible errors made while assessing the parameters, Table 4 contains a comparison of results relating to determination of coefficients of permeability (Cp) from the data of pumping thermal waters from certain wells in the Tobol region of the Tjumen area. In the majority of cases, thermal waters are saturated with gas. On the way from the aquifer to the well mouth the pressure in the water decreases and at a certain depth, corresponding to the pressure of satu1178
1.30 0.40 0.70
27,
-- level decline in the aquifer; Se - - decrease in the borehole face pressure; AP~ - - decrease in the pressure at the mouth of the well; AP,x y,,y,,,yd*- volume weight of water in the aquifer at static and dynamic levels (or at the point of outflow, in the case of the producing wells), respectively.
S
1 2 3
(2)
where
H
Coefficients of permeability, C~ Without taking into Taking into account the account the difference difference in temperature in temperature
"=ca --
C
(3)
- - actual radius of an exploratory well: - - radius of an equivalent fully penetrating well;
C
- - c o e f f i c i e n t characterizing additional filtration resistances by partial penetration of well according to the degree and character of the acquifer tapping.
This coefficient is determined from the theoretical and empirical relationships and graphs. The term ~ exploitable reserves of subterranean waters ~ means their yield, which can be obtained by operating the efficient (in technical and economic respects) water intakes under the given regime of exploitation and water quality, satisfying the standards during the whole period of water use. Therefore, assessment of the exploitable reserves actually implies calculation of the indices relating to water intakes, i.e., determining the number of wells and their locations, yields and dynamic level declines, water quality and changes in these indices with time.
Assessment of exploitable reserves of subterranean waters is accomplished by (1) hydrodynamic, (2) hydraulic and (3) balance methods which have been described by B1Nt)EMAN (1963). The hydrodynamic method is generally employed for estimating the reserves of subterranean thermal waters in the platform areas, whereas the hydraulic and balance methods are used to assess the thermal water deposits of the fissure-vein type. Application of the above methods has been dealt with in works by GOLDBERGand YAZVIN (1965), PLOTNIKOV (1959, 1968), BOCHEVER (1968), BOCHEVER and VERIGIN (1961), BINDEMAN (1963, 1964) and others. A number of accurate and complex solutions for the cases with heterogeneous water-bearing rocks and conditions of interconnection of separate aquifers with the exploited one, have been considered by MASKET (1948), SHCHELKACHEV (1959) and BOCHEVER (1968). Therefore, these methods and solutions are not considered in this paper. Prospecting of fissure-vein thermal water deposits is usually conducted within the confines of the previously discovered thermal anomalies: the discharge sites of thermal waters of fissure head-water systems in folded areas. At the reconnoitring stage of exploration, it is expedient in the majority of cases to conduct a large-scale hydrogeological survey in the area of the deposit, accompanied by geophysical and thermometric investigations, by using boreholes for map compilation and by well-testing to study the geological and structural conditions, the composition and temperature of thermal waters, as well as approximate thermal resources. At the stage of preliminary and detailed prospecting, the exploratory wells are drilled to bring the natural reserves of thermal waters or steam to the surface in the zones of tectonic disturbances revealed by the survey or prospecting boring. The number of prospecting wells and their depths depends on the size of the deposits and their geological and structural peculiarities. To bring to the surface the natural reserves of thermal water deposits confined to the granite massifs or zones of crystalline, metamorphic rocks, it is usually enough to drill from 3 to 5 prospecting wells. In the areas of recent volcanic activity (usually characterized by large deposits of thermal waters or steam as compared to granite massifs), the number of prospecting wells may amount to 10-30 and even more. The installation of each well is followed by a series of geophysical investigations (coring by lateral sounding, gammalogging, thermal logging, inclinometry) to determine the main zones of thermal water inflows. As has been proved by prospecting of fissure-vein thermal water deposits, the prospecting wells should be spaced at 300-500 m to avoid strong interaction. In this case, the coefficient of interaction does not usually
exceed 0.2-0.3. When interaction accounts for up to 50 or more per cent of the yield of an individual well, the cost price of thermal water extraction sharply increases. Accumulated experience of drilling prospecting wells in fissure-vein thermal water deposits located beyond the regions of recent volcanic activity, makes it possible to recommend boring down to a depth of 200300 m. Individual wells can have a maximum depth of no more than 500 m. In the areas of recent volcanic activit~v the depth of prospecting wells should not exceed 300-500 m (seldom 600-700 m). Individual prospecting wells may be drilled down to a depth of 1000 m. The above depths ensure tapping of the main tectonic fissures with the original flows of thermal waters or steam. It should be made clear that the deposits in question are represented by the discharge areas of thermal waters with the generally observed ascending flows responsible for the coincidence in the directions of both the filtrational and thermal streams, and a decrease in the geothermal gradient with depth (in conformity with the exponential law), which results in the fact that the water temperature at a certain depth quickly approaches the maximum and practically does not change any more. A decrease in temperature with depth is frequently observed in the wells. The decrease is connected with the moving away from the vent fissures, hit at certain depth intervals. In the upper part o'f the profile composed of loose rocks, wells are drilled with the use of mud solutions. On casing and cementation, the lower part of the profile with solid walls is drilled and the well is flushed with clean water. When prospecting deposits of steam it is essential to have a pumping plant (with a capacity of 25-30, or more, I/see) to cool the borehole and prevent steam and water blow-outs. The temperature of the outflowing flushing liquid must not exceed 80-85 °C. On completion of drilling, screen columns with perforated tubes at the points of productive intervals are lowered into the prospecting wells. This is followed by a long-term exploratory release of thermal waters from a group of wells with maximum yields until stabilization of discharge and corresponding decline in the dynamic level are achieved. Low productive and unproductive wells serve as the observation ones. For instance, on the Pauzhetka deposit of steam, 10 prospecting boreholes were productive and 11 served as observation ones. The duration of an exploratory release of thermal waters or steam may differ (e.g., 3-5 months to one year). It is minimal on small deposits located in granite massifs and maximum on the deposits of steam. If the water levels in the observation wells do not stabilize for a long time, but, on the contrary, gradually decline under a constant yield of the water intake, this can indicate that the release of thermal water 1179
(steam and water mixture) exceeds the natural reserves, and it is necessary to decrease the water discharge by cutting off a number of wells or shutting the slidevalves. A considerable drop in water temperature and changes of water composition in the marginal wells are also indicative of the drawing up of surrounding cold waters. An exploratory release makes it possible to assess the true volume of exploitable reserves of the deposit (site) as well as stability in time of water composition and temperature. Such a release crowns prospeering works, and its results serve as the basis for the determination of the exploitable reserves of the deposit, elaboration of certain standards (quantity and quality of water or steam and water mixture, conditions of exploitation, indices of economic effectiveness) and compilation of the scheme for water intake exploitation based on the parameters determined in the course of prospecting. The geological-commercial assessment of the various deposits of thermal waters or steam presents some difficulties due to the various types of utilization (heating of dwelling houses, greenhouses, hotbeds, etc; hot water supply; in technological processes of washing, drying, fermentation, etc; in Russian baths, shower baths, laundries, etc.; in balneology; for filling swimming pools; in the extraction of chemically valuable components; for electric power generation in a number of regions of recent volcanicity). The possibility of such a multipurpose utilization of thermal waters hampers elaboration of the technical and economic indices and determination of the economic efficiency of their utilization. Complex utilization of thermal waters by a certain group of consumers is the main prerequisite for an efficient exploitation of their deposits, raising profitability and promoting a more complete use of the potential heat. Since thermal waters are regarded as a new source of thermal energy, the geological-commercial assessment of their deposits should be generally based on the above aspects. Geological - Economical water deposits
assessment
of
thermal
Up till the present moment, the deposits of thermal waters from the viewpoint of their utilization have been mainly assessed by taking into account their hydrogeologieal, therrnodynamie and power generation aspects (1). Unfortunately, investigations conducted in this field did not deal sufficiently with the problems relating to the economics of reconnoitring, prospecting and utilization of thermal water deposits, though the geological-economic analysis should be regarded as one of the main elements of geological prospecting, primarily aimed (1) This paper does not deal with the geological-eeonomical assessment of natural steam deposits intended for electric power generation. 1180
at establishing their relative significance for the people's economy and priority of commercial development. Hydrogeologists are confronted with the above problems at all stages of prospecting until the very moment of utilizing the deposit for commercial development. Underestimation of the geological-economic assessment results in a detailed prospecting of deposits which presents no value for industry now and will be useless in the future. It goes without saying that at various stages of geological prospecting the purposes of the geological-economic assessment will be quite different, as every stage pursues its own objects. At the stage of reconnoitring and prospecting it is already possible to have a sufficiently correct view of the commercial type and amount of the thermal water deposit and of its quality. Hence it is possible to employ not only hydrogeological but also economic methods of assessment. Economic assessment at this stage helps to establish the potential value of the deposit and to come to a conclusion on the expediency of prospecting. Such assessment should be based on the minimal number of technical and economic indices relating to the reserves and quality of thermal waters. The parameters to be assessed are as follows: 1)- Possible magnitude of reserves. 2) - Their quality.
3)- Conditions of occurrence and formation of subterranean thermal waters. 4) - Tentative value of the deposit in terms of money, etc. The geological-economic assessment at the stage of preliminary prospecting of a thermal water deposit should concentrate on the solution of problems relating to the exoediency of its development. The geological-economic assessment based on the data of preliminary prospecting should characterize the future raw material base. It should elucidate the natural and economic factors, which are of great importance for selecting a variant of the deposit development, and contain an analysis of the effect of these factors on the selection of the main technological solutions. On the basis of all the results obtained from the preliminary prospecting, the technical-economic substantiations or reports (TES or TER) are compiled. These reflect the economic expediency and establish the priority of commercial development of the given deposit. While compiling TES or TER, the following technical and economic indices should be taken into account:
Natural value of the deposit a) - reserves of thermal waters; b)- useful comvonents (thermal capacity, balneologieal significance, etc.); c) - value of the deposit in terms of money.
EHiciency o[ the exploitation a) b) c) d)-
possible annual capacity of wells; cost price of exploitation (with regard to extraction); profitability of exploitation; level of profitability in per cent to basic assets.
EHectiveness o[ capita investments a)- capital investments on construction of water intakes; b) - specific capital investments; c) - the term of justifying the capital investments (with regard to main objects); d) - level of profitability in relation to the invested money. Thus, the assessment indices (taking into account the purpose and objectives of preliminary prospecting) are calculated with regard to the total reserves of the deposit of thermal waters without detailing for separate blocks, sites and aquifers. In its concluding part, the TER contains data on the relative commercial value of the deposit and recommendations for the carrying out of detailed prospecting. At the stage of detailed prospecting it is significant to determine: I) - The possible consumers (depending on the properties of thermal waters and presence of useful components) which are essential for the selection of the best variant of utilization of thermal waters requiring minimum investments. 2) - Reserves of thermal waters and average capacity of wells. 3) - Amount of investments and specific capital expenditures on the deposit development. 4) . Cost price of a unit of the finished product (at the stage of extraction and consumption). 5) - Probable income to be derived, and the term of justification of the invested money. In other words, the technical and economic indices calculated at the stage of preliminary prospecting are further specified at the stage of detailed prospecting. Besides, the assessment indices should now be determined not only for the thermal water deposit as a whole, but also for separate sites (aquifers, blocks, parts of water intakes, etc.), and such indices as the cost price of the finished product, probable income to be derived, and capital investments. Their efficiency and terms of justification should be calculated at the stage of extraction and at every subsequent stage up to utilization by the consumers. Numerical expression of every assessment index is found through calculation. The initial figures should be obtained on the basis of a profound and apprehensive investigation of the particular conditions of the thermal water deposits. It should be noted that the geological-economic assessment at the stage of prospecting should be carded out in a complete way, taking into account all the useful properties of thermal waters, i.e. the criteria to be assessed are as follows:
1). Hydrogeological quantitative and qualitative characteristics of the deposit. 2)- Hydrogeological, technological and heat engineering factors: the method and system of exploitation of the deposit (spouting or pumping off), the possible capacity of the wells and the scheme of their locations, the rational " amount of wells in a water intake, the various possible uses of the thermal waters, etc. These criteria (taking into consideration the conditions of the deposit) constitute the basis for establishing the above enumerated economic indices. The geological-economic assessment carried out at the stage of detailed prospecting is very significant as it determines the future of the deposit. That is why it should be accomplished by a number of specialists, viz., hydrogeologists, economists, experts in heat engineering and power generation, agronomists, balneologists, etc. While assessing a deposit of thermal water, it is logical to consider utilization of thermal waters from the moment of extraction till the moment of supply (through pipelines) to the consumers and to make the assessment according to the finished product (1 cal of heat, 1 kWh, 1 ton of iodine, 1 ton of vegetables, etc.) and not only according to the cost of extraction of 1 m s of thermal water (the half-finished product in this case). This is very essential for planning capital investments in the industries utilizing useful properties of thermal waters. Evaluation of a deposit according to the finished product can help to assess unprofitability correctly, or economy of the thermal water intakes. The cost price of a unit of finished product reflects a number of indices such as level of investments. exploitational expenses, capacity of separate wells and heat potential. The cost price of 1 m 3 of thermal water is found from the formula: tl
C., = v_ k~"~i + E.Ce + W , . N + 0 1
where C . . . . cost price of 1 m 3 of the extracted water; ki - - c a p i t a l investments per I m 3 of water; ~i - - deduction from the basic assets (taking into account the normal coefficient 0.15); E - - e x p e n d i t u r e of electric energy on extraction of 1 m 3 of water; Ce - - c o s t
of 1 kWh of electric energy;
W p - - w a g e s of the personnel; N - - n u m b e r of workers; O - - o t h e r expenses on extraction of 1 m 3 of water amounting to 10 per cent of the exploitational expenses; Table 5 contains data on the cost price of 1 m 3 of thermal waters and 1 kcal of heat at the stage of deposit prospecting. 1181
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Determination of the cost price of 1 m 3 of thermal water and 1 kcal of heat was based on the following assumptions: 1)- When carrying out prospecting of fissure-vein or stratal thermal water deposits, the depths of wells do not exceed 300-500 and 1500-2500 (seldom more) metres respectively. 2) - Deposits of the first type are bored by column drills and those of the second type by rotary drills. 3) - The amount of the obtained heat was determined taking into account the lower temperature limit of water 200 C 0). 4 ) - The yields of individual wells (as has been proved by experience in prospecting and developing thermal water
deposits) fluctuate from 5 to 20 or more l/see, and the water temperature at the well mouth generally fluctuates from 40 to 90°C. Therefore, while calculating the cost price of 1 m 3 of water and 1 kcal of the obtained heat, we have assumed the followin~ yield values of the individual wells of a water intake Q): (a) for spouting wells: 5, 10. 15 and 20 1/sec; (b) fbr pumped wells: 10, 20, 30 1/see.
Besides, we have determined the influence of interaction of wells in a water intake (at the interaction coefficients of 0.2, 0.5, 0.7) on the cost price of water. A high coefficient of interaction is most frecluentlv observed in prospecting the fissure-vein deposits. Therefore, the changes in the cost price have been calculated for this type of deposit only. Table 5 contains all the data relating to the calculations of the cost price of extraction of 1 m 3 of thermal water and for obtaining 1 kcal of heat in the USSR's three most typical regions of thermal water occurrence: the north Caucasus, Uzbekistan and west Siberia. These data were compared with the tariffs on thermal energy in the regions in question. A thick line separates those conditions of prospecting under which the cost price of I kcal does not exceed the tariffs of thermal energy. Table 5 demonstrates that the cost price of 1 kcal of heat is below that envisaged by the tariffs for the wells 300-500 m deep, with a yield of 5 and more l/see and a water temperature equal to, exceeding, 40 °C. If a well is 1500-2000 m deep, the cost price of the obtained heat is below the tariff one at a yield of 15 1/see and water temperature exceeding 70°C. When the wells are 2500 m deep the respective figures are I0 I/sec and 80 °C. It should be pointed out that the pumping of thermal waters, while increasing the cost price of 1 m 3 of water and I kcal of heat somewhat, considerably increases the heat potential of a strata1 thermal water deposit. Moreover, a smaller number of wells may be required to provide for the same thermal loading; therefore, the possibility of deep pumping must be consid(D Geothermal heat is mainly used by consumers for heating and hot water supply. In the first case the lower temperature limit is assumed to be 40°C and in the second, 5°C. We have assumed an average temperature limit of 20°C. C-) For the calculations it was conditionally assumed that the water intake consisted of 5-8 wells.
ered when elaborating projects on exploitation of deposits. One should bear in mind that a comparison of thermal heat cost price with the tariffs of thermal energy makes the performed calculations more reliable. Table 5 contains the tariffs on heat supplied by large central heating plants or district boiler houses, whereas the geothermal heat is generally utilized in ~mall capacity plants intended for a non-centralized heat consumption. Therefore, the cost price of 1 kcal of heat produced by the latter is twice as large. The calculations prove that the cost price of a unit of produce increases by 2-3 times where the coefficient of interaction of wells changes from 0.2 to 0.7. Up till the present moment, thermal water deposits have been assessed (if at all) according to their socalled <) property, heat, even though in most of the cases, heat is not the only valuable property of thermal water deposits and this necessitates their complex utilization. REFERENCES
]~HHAeMaH H. H. 1963. OtlteHKa aKCnhyaTaI.IMOHHraIX 3al-laCOB nOA3eMHbIX BOA. FoczeovexuaOav. BnHAeMaH H. H., BoqeBep ~ . M. 1964. PerrioHaAbHa~[ oIleHKa 3KCrIAVaTauriOHHblX 3ariaCOB IIOA3eMHblX BOA. Hec)pa. Moc~coa. Boqeaep ~ . M., BepHrnH H. H. 1961. MeTOArI~eCKOe noco6rie no p a c q e T a m :~KCrIAyaTaIII,IOHHHX 3 a n o c o B HOA3eMHblX BOA AA$I BOAOCHa6>KeHH~.
Focyc)apcveennoe uac)arenz,reo nuTeparypt, t no crpourenbcvay, apxurewrype u crpourem,nbt• a~arepua.aaa. Moc~coa. BbIxoaep H. A., Ka6axn•ae A. rI., Certarrig F. II. rt Ap. 1968. :gKortoMriKa UriHepaAbnoro cr,xpb a r t reoAoro-paaaeAoqHblX pa6oT. /430. Hec)pa. FOAb6epr B. M., ~Iamm A. C. 1965. MeToAHqecKHe yKa3anna rio oUeHKe aKCnAyaTaI~HOHHbIX 3anacoB TepmaAbHblX BO,A,. I/I30. /4HCTUTt4Ta BCEFHHFEO. MaBprmKHh B. ~ . , AHTOHeHKO F. K. 1967. Onr:,lT HCCAeAOBaHH~I,
paaBeAKn
H
HCHOAIx3OBaHI.I~
B npaKTHqecKnx iaeA~iX TepMaAI~HI,IX BOA a CCCP I4 3a p¥6emoM. HeO.t)a. MaBpHIJKHI,~ ]~. 0:15. 1967. HeKoTopbm BorrpocI,I qbopMHpOBaHHSl TepMaAbHi,IX BOA, H IlepClIeKTrlBbl HK HClIOAb3OBaHH~I. Coeerczea.q zeonozun.
MaCKeT T. 1948. TeqeHae OAHOpOAH~,IX mHAKOCTefi B nopncTO~ cpeAe, rocronrexu3c)ar. rIAOTHHKOB H. A. 1959. OtleHKa 3anacoB nOA3eMHI~IX BOA. Foczeonvexuac)ar. IIAoTHrlKOB H. H. 1968. IIOnCKH ~I paaaeAKa npeCHI~IX IIOA3eMHIaIX BOA AA~;I IIeAefi K p y r i H o r o
BOAOCna6x~enrlst. ~t. FI. uab. MFY. 1967. PerrtoHahi, Ha~ reoTepMi~a H pacrlpocTpaHenrle TepMaAI~HI,IX BOA B CCCP. TO. a o o p o r o coBem.iIO reoTepMrtqecKriM ~ICCAeAOBaHIa~M B CCCP. H30. Hav~ca, M. II.l~eAKaqeB B. hi. 1959. x]'nlaVr~Ifi pe>KnM rtAac'rOBblX BOAOHarlOpHlalX CHCTeM. FOCTOnTexu3c)aT.
1183
Methods for Exploration of Thermal Waters and a GeologicalCommercial Assessment of Their Deposits S. S. BONDARENKO*, B, F. MAVR1TSKY * AND L. F. POLUBOTKO *
ABSTRACT The concepts of the regularities of thermal water occurrence, the conditions of their occurrence and circulation and hydrogeothermal indices of thermal water deposits are the basis of the methods of thermal water exploration. According to the conditions of occurrence and circulation all thermal waters are divided into two types: stratal and fissure-veined waters. Stratal thermal waters occur in platform regions, foredeeps and intermontane areas, and extend to m o s t large areas (tens and hundreds of thousands of square kilometers). The fissure-veined thermal waters occur ,in folded areas and have a local nature of outlet and extend to small areas (from portions to units of square kilometers). The methods of exploration and determination of the exploitation resources of thermal water deposits of stratal and fissure veined type are different. Based on the analysis of the available geological, hydrogeological and geothermal data on the basins in which stratal thermal water deposits are found, the principal aquifers and complexes with thermal water of usable quality for heat supply are determined and distinguished. Exploration of these aquifers, and complexes distinguished, is conducted for determination of calculating parameters required for calculation of exploitation resources. These resources are determined by means of hydrodynamic computation, taking into account the boundary conditions (infinite bed, semi.finite bed, bed-strip, etc.). Exploration of the fissure-veined thermal water deposits is carried out in previously established thermal anomalies using the ground geophysical methods and the complex hydrogeothermal survey at a large scale, by means of which the structure and sizes of a deposit, composition and temperature of thermal waters (superheated waters) and the approximate possible heat quantity of a deposit are determined. The preliminary data obtained are made more precise by drilling of exploration wells, and the deposit resources are estimated by the method of pumping tests of thermal waters fronf a group of wells. The hydraulic method is the principal one for determining exploitation resources of the fissure-veined thermal waters. In conducting exploration works at the thermal water deposits (and superheated waters) the main technical-economical indices are defined. The question of industrial estimation of the thermal water deposits arises before the hydrogeologists at all stages of their exploration, from the stage of prospecting works to their exploitation for industrial purposes. The economic significance of thermal water deposits, and economic efficiency in their exploitation are established on the basis of geological-economic estimation. The above methods enable the reseacher to plan the succession of development of thermal water deposits. The application of methods for reconnoitring and exploration of minerals depends on the peculiarities of their origin, regularities of distribution and conditions of occurrence. Elucidation of the geological situation * Union Research Institute of Hydrogeology and Engineering Geology (VSEGINGEO), Moscow, USSR.
and hydrothermal conditions which influence the formation of thermal water deposits is a prerequisite for selecting and substantiating the methods and direction of research and exploration of thermal waters and steam. Investigations, which have been carried out by various geological organizations of the USSR Ministry of Geology and the USSR Academy of Sciences, have made it possible to establish the general regularities of thermal water and steam occurrence within the confines of the Soviet Union, and reveal prospective regions where these waters can be utilized for electric power generation, introduction of district heating plants, balneology, etc. (GOLDBERG, YAZRIN 1965; MAVRITSKY,ANTONENKO 1967; PLOTNIKOV 1968). According to the conditions of occurrence and character of circulation, the thermal waters of the prospective regions are subdivided into stratal and fissure-vein. The former occur in vast areas within the confines of Epihercynic platforms, foredeeps and intermontane depressions composed of the Mesozoic and Cenozoic sedimentary deposits, and are predominantly united to form large head-water systems. The latter are characterized by local distribution and are generally confined to the places of intersection of large fractured zones with deep erosion cuttings. These deposits are connected with head-water systems in the discharge areas of the fissuretype. An approximate subdivision of the deposits of thermal waters and steam into types is given in Table 1. The deposits of thermal waters in platform areas may be rather considerable, which calls for the singling out of exploitable sites (within the confines of such deposits) where it is expedient to utilize them, An accurate determination of the boundaries of thermal water deposits in a number of regions is hampered by the gradual change of their parameters (temperature, salt content and chemical composition of subterranean waters, thickness and depth of water-bearing rocks, etc.). In such cases (for instance, in the West-Siberian artesian basin) the boundaries of deposits are only roughly outlined. The scope of hydrogeological prospecting is mainly dependent upon the amount of knowledge concerning the particular deposit accumulated during the period of 1175
TABLE 1. Type of deposits
'~ "~
-
-
Classi/ication o/ the deposits o/ thermal waters.
Class of deposits (according Kinds of deposits to water temperature) (accordingto salt content)
Character of flow
Hydrodynamic features
1. Low-thermal (from 200 to 50 °C)
I. Fresh (up to 1 g/l)
1. Flowing, unsaturated with gas
1. Continuous stratum
2. Thermal (from 500 to 100 °C)
2. Brackish (1-10 g/l)
2. Flowing, saturated with gas
2. Semirestricted stratum
3. Highly thermal (more than 100 °C)
3. Saline (10-35 g/l)
3. Eruption of steam and water mixture
3. Stratum-band 4. Stratum-quadrant
1. Low-thermal (from 20o to 50 °C)
1. Fresh (up to 1 g/l)
1. Flowing, unsaturated with gas
2. Thermal (from 50o to 100 °C)
2. Brackish (!-10 g/l)
2. Flowing, saturated with gas
3. Highly thermal (more than I00 oC)
3. Saline (10-35 g/l)
3. Eruption of steam and water mixture
LL
time preceding direct investigations. In order to make reconnoitring and exploration more efficient, they should be conducted by stages and in strict succession. The aim of the first stage of exploration is, first of all, to reveal a deposit of thermal waters and single out a site (or several sites) prospective from the viewpoint of extracting and utilizing these waters in accordance with hydrogeological, technological and economic factors. The search for thermal waters can be subdivided into two stages. The first stage involves the study of the geological structure of the territory; the general regularities of distribution of subterranean thermal waters; depths and lithology of aquifers; chemical and gas composition of subterranean waters. This study should be based on the results of the preceding explorations, hydrogeological as well as deep test drilling and boring for oil. At this stage the predicted resources of thermal waters of the deposit are assessed and the hydrogeological conditions and possible resources of thermal waters on the revealed prospective sites are compared, with the object of developing them immediately. The second stage, exploration boring, involves investigation of the geological profile of rocks composing the deposit, determination of depths and thicknesses of aquifers, collector properties of water-bearing rocks, chemical and gas composition of thermal waters, heads of the subterranean waters and a study of the tapped aquifers by pumping. The exploration work is also aimed at specifying the depths and thicknesses of the water-bearing complexes and aquifers, estimating their water resources, and investigating the chemical composition and regime of subterranean waters. 1176
1. With one thermal water producing tectonic fissure (zone) 2. Block-like structure (a number of tectonic, mutually intersecting fissures)
The calculated and specific yields of wells, depths of dynamic levels (decreases in heads) of subterranean waters in the wells over the period of exploitation, optimum exploitational regime of the wells and deposit (or part of it) must be determined based on an analysis of the investigation results. Study of the standard demands on the deposit of thermal waters and the estimation of their exploitable resources are the purpose and most significant task of reconnoitring and prospecting. Experience shows that inadequacy of the revealed exploitable resources considerably prolongs the period of development and increases expenditures on preparation of the deposit for commercial use. During the period of prospecting it is obligatory to define the size of the exploitational site, as it determines the volume of exploitable suberranean water resources and the number of exploitational wells. Analysis of the hydrodynamic situations of deposit exploitation has proved the necessity of limiting the areas of exploitable sites under platform conditions, to ensure the most efficient exploitation of subterranean water resources. An unjustified increase in the area of the site under exploration will bring about an appreciable increase in the cost of prospecting work, and will result in an insignificant yield of the exploitable resources of subterranean thermal waters, which is stipulated by the peculiarities of distribution and regime of the subterranean thermal waters within the confines of large geological structures. An elastic-pressing filtration regime of subterranean waters is characteristic of the majority of thermal water deposits in artesian basins. The collector properties of the deeply seated aquifers are usually characterized by small values of hydraulic conductivity (generally less than 1 m/day) and water transmissibility (from several
dozen to 1()0-150 and seldom more m2/day). T h e initial yields of i n d i v i d u a l wells (especially u n d e r excess of head) can be considerable. They will, however, decrease w i t h time due to the d e v e l o p m e n t of a depression cone and, mainly, owing to the interaction of wells during their exploitation. The discharges of water intakes usually fluctuate from 50 to 200-300 1/sec ( T a b l e 2). The above considerations necessitate a c o m p l e x assessment of thermal w a t e r deposits taking into account the technical and economic indices relating to extraction and utilization of thermal waters in concrete conditions a n d regions. Assessment of the exploitable resources of thermal waters on the site of the w a t e r intake u n d e r design in p l a t f o r m conditions includes: •
A wealth of special publications deals w i t h the d e t e r m i n a t i o n of p a r a m e t e r s of water-bearing rocks. Therefore, there is no need to consider this subject in detail.
)
1)- Evaluation of the calculated hydrogeological parameters of the water-bearing rocks inside and outside the water intake site according to the data provided by boring and exploratory hydrogeological testing of wells. 2) - Schematization of the analysed hydrogeological conditions in the zone of possible influence of the water intake and elaboration of the basic calculation of the hydrodynamic scheme. 3) - Expression in figures of the standard demands on thermal waters and conditions of their exploitation by accomplishing consecutive hydrodynamic and technico-economic calculations based on the variational method. 4) - Calculation of the total yield of the water intake (taking into account the standard demands) with respect to the most rational system (number and distribution) of the exploitational wells. This yield is regarded as the exploitable reserves of subterranean thermal waters. 5) - Classification of the exploitable resources according to the availability of investigational data for the particular site, trustworthiness and degree of reliability of the initial parameters.
T h e accuracy of p a r a m e t e r s of deep-seated aquifers is affected by a n u m b e r of factors, including: a ) - changes of teraperature in the borehole during different periods of exploratory hydrogeological investigations; b) - gas saturation of subterranean waters; c ) - hydrodynamic imperfection of a well; d) - resistance to the water movement in tubes on their way from the aquifer to the mouth of the well. These mainly decrease the dynamic levels of excessive pressure at the m o u t h of the flowing wells. U n d e r e s t i m a t i o n of the above factors m a y entail grave errors in determining p a r a m e t e r s of water-bearing rocks and exploitable resources of s u b t e r r a n e a n waters. The t e m p e r a t u r e and, consequently, w a t e r volume weight changes in a borehole are d e p e n d e n t on both the value of geothermal gradient and the t e m p e r a t u r e on the land surface. The difference is the greatest between the temperatures of w a t e r in the aquifer and at the static level (or at the mouth) when p u m p i n g has not been effected. This is the case w h e n static levels or excess pressures are measured in the test wells or static and dynamic levels (pressures) in the observation ones. In the course of p u m p i n g (test release) the w a t e r temperature at the m o u t h of the well d e p e n d s on the w a t e r t e m p e r a t u r e in the aquifer and w a t e r velocity in the borehole, i.e., on the yield. Therefore, the average volume weight of water in a borehole w h e n measuring
T^aL~ 2. - - Characteristics o~ thermal water deposits. Deposits
Kolpashevsk BarabainskKupinsk Tashkent
Makhasehkala
The main thermal water-bearing complexes
Approxi. mate depths of wells, m
Salt content, g/1
Neocomian and Apt-Cenomanian -- ~ --
1100-2200
1-5
1100-2000
1-15
Cenomanian
1500-2200
Plains o/ Uzbekistan 1-3 up to 150
Kizlyar
Chokrakkaragansk -- ~ --
1000-3000 2200-3000
Saki-Evpatoria
Neocomian
1000-1700
Excessive heads, m
t ° of water Piezoconat the point ductivity, of out-flowing mZ/day
Average wa- Calculated ter trans- yields of a missibility standard mZ/day water intake, 1/see (1)
West Siberia 20-50
35-75
105
190
150-250
10-30
35-65
105
200
130-250
50-75
5.105
Eastern Predkavkazje 2-20 20-50 10-20
100
The Crimean plains 5-10 up to 120
30
up to 60-70
45-90
5.105
250
300-400
80-100
105
150
up to 300
40
50-70
50-60
5.105
(~) A water intake with an area of 25 km z and 5 well.
The calculated time of exploitation: 104 days; level decline: 100 m below the earth surface. 1177
the static level will exceed that when measuring the dynamic levels in the process of pumping. In the first approximation, a level decline in the aquifer will be equal to:
T^aLE 4. -- Examples o~ the possible errors made while assessing coe[[icients o[ permeability due to changes in tem. perature.
Wells a) in a producing well Sd --
-~o.-
At,!
- - H Y'' ~
yat + 1 0 AP,_____~x
2ys
7,
(1)
y,
b) in a non-producing well Sd
- -
y,t--Ydt
+S
2Y,
7dl+Ys
-- head of subterranean waters in the well (in the case of a flowing well, the depth to the confining bed of the aquifer): --measured level decline, in m.
Table 3 contains the calculated values of errors AS (when determining a level decline Sd) dependent on the temperature at the well mouth before pumping out (t%0 and in the process of flowing (t°g) for the water with a salt content of 20-25 g/1. TABLE 3.
The calculated values o/ errors A S depending on the temperature ]'actor. -
-
Temperature Temperature at the sur- at the surface (t°.,) face during pumping out (t°,) I0
20
50 55 60 65 70
50 55 60 65 70
y.,
y,
H (m) 1000 [ 1600
0.76 0.28 0.54
4
1.48
0.18
Average
1.22
0.44
ration for the given volume and composition of the saturating gas, the gas starts escaping from the water. The specific weight of the formed mixture of gas and water is smaller than that of water without gas. Therefore, the fixed level decline (pressure) will not fully correspond to the decrease in the formational pressure. For the same conditions of the Tobol region, Tjumen area, an additional water level decline (A S) at a salt content of 18 g/l, gas factor 1.0, pressure of the saturating gas 68 atm, is 40.7 m (at P excessive = 1 atm), 32.2 m (at P,x = 2 atm), etc., i,e., the value of A S is comparatively high even at small values of the excessive pressure and gas factor. Partial penetration of wells (according to the degree and character of aquifer tapping) is responsible for additional resistance and head losses on the water movement path directly in the face zone and in the screen; this considerably decreases their water capacity. That is why the formulae used for determining the parameters contain the radius of an equivalent, fullypenetrating well instead of the actual one:
A S per A S per 1000 m 1600 m 1.0144
1.0125
1.0016 0.9993 0.9967 0.9948 0.9928
6.4 7.5 8.8 9.8 10.8
10.2 12.1 14.2 15.7 17.3
1.0016 0.9993 0.9967 0.9948 0.9928
5.5 6;6 7.9 8.9 9.8
8.7 10.5 12.6 14.2 15.8
By way of an example to illustrate the possible errors made while assessing the parameters, Table 4 contains a comparison of results relating to determination of coefficients of permeability (Cp) from the data of pumping thermal waters from certain wells in the Tobol region of the Tjumen area. In the majority of cases, thermal waters are saturated with gas. On the way from the aquifer to the well mouth the pressure in the water decreases and at a certain depth, corresponding to the pressure of satu1178
1.30 0.40 0.70
27,
-- level decline in the aquifer; Se - - decrease in the borehole face pressure; AP~ - - decrease in the pressure at the mouth of the well; AP,x y,,y,,,yd*- volume weight of water in the aquifer at static and dynamic levels (or at the point of outflow, in the case of the producing wells), respectively.
S
1 2 3
(2)
where
H
Coefficients of permeability, C~ Without taking into Taking into account the account the difference difference in temperature in temperature
"=ca --
C
(3)
- - actual radius of an exploratory well: - - radius of an equivalent fully penetrating well;
C
- - c o e f f i c i e n t characterizing additional filtration resistances by partial penetration of well according to the degree and character of the acquifer tapping.
This coefficient is determined from the theoretical and empirical relationships and graphs. The term ~ exploitable reserves of subterranean waters ~ means their yield, which can be obtained by operating the efficient (in technical and economic respects) water intakes under the given regime of exploitation and water quality, satisfying the standards during the whole period of water use. Therefore, assessment of the exploitable reserves actually implies calculation of the indices relating to water intakes, i.e., determining the number of wells and their locations, yields and dynamic level declines, water quality and changes in these indices with time.
Assessment of exploitable reserves of subterranean waters is accomplished by (1) hydrodynamic, (2) hydraulic and (3) balance methods which have been described by B1Nt)EMAN (1963). The hydrodynamic method is generally employed for estimating the reserves of subterranean thermal waters in the platform areas, whereas the hydraulic and balance methods are used to assess the thermal water deposits of the fissure-vein type. Application of the above methods has been dealt with in works by GOLDBERGand YAZVIN (1965), PLOTNIKOV (1959, 1968), BOCHEVER (1968), BOCHEVER and VERIGIN (1961), BINDEMAN (1963, 1964) and others. A number of accurate and complex solutions for the cases with heterogeneous water-bearing rocks and conditions of interconnection of separate aquifers with the exploited one, have been considered by MASKET (1948), SHCHELKACHEV (1959) and BOCHEVER (1968). Therefore, these methods and solutions are not considered in this paper. Prospecting of fissure-vein thermal water deposits is usually conducted within the confines of the previously discovered thermal anomalies: the discharge sites of thermal waters of fissure head-water systems in folded areas. At the reconnoitring stage of exploration, it is expedient in the majority of cases to conduct a large-scale hydrogeological survey in the area of the deposit, accompanied by geophysical and thermometric investigations, by using boreholes for map compilation and by well-testing to study the geological and structural conditions, the composition and temperature of thermal waters, as well as approximate thermal resources. At the stage of preliminary and detailed prospecting, the exploratory wells are drilled to bring the natural reserves of thermal waters or steam to the surface in the zones of tectonic disturbances revealed by the survey or prospecting boring. The number of prospecting wells and their depths depends on the size of the deposits and their geological and structural peculiarities. To bring to the surface the natural reserves of thermal water deposits confined to the granite massifs or zones of crystalline, metamorphic rocks, it is usually enough to drill from 3 to 5 prospecting wells. In the areas of recent volcanic activity (usually characterized by large deposits of thermal waters or steam as compared to granite massifs), the number of prospecting wells may amount to 10-30 and even more. The installation of each well is followed by a series of geophysical investigations (coring by lateral sounding, gammalogging, thermal logging, inclinometry) to determine the main zones of thermal water inflows. As has been proved by prospecting of fissure-vein thermal water deposits, the prospecting wells should be spaced at 300-500 m to avoid strong interaction. In this case, the coefficient of interaction does not usually
exceed 0.2-0.3. When interaction accounts for up to 50 or more per cent of the yield of an individual well, the cost price of thermal water extraction sharply increases. Accumulated experience of drilling prospecting wells in fissure-vein thermal water deposits located beyond the regions of recent volcanic activity, makes it possible to recommend boring down to a depth of 200300 m. Individual wells can have a maximum depth of no more than 500 m. In the areas of recent volcanic activit~v the depth of prospecting wells should not exceed 300-500 m (seldom 600-700 m). Individual prospecting wells may be drilled down to a depth of 1000 m. The above depths ensure tapping of the main tectonic fissures with the original flows of thermal waters or steam. It should be made clear that the deposits in question are represented by the discharge areas of thermal waters with the generally observed ascending flows responsible for the coincidence in the directions of both the filtrational and thermal streams, and a decrease in the geothermal gradient with depth (in conformity with the exponential law), which results in the fact that the water temperature at a certain depth quickly approaches the maximum and practically does not change any more. A decrease in temperature with depth is frequently observed in the wells. The decrease is connected with the moving away from the vent fissures, hit at certain depth intervals. In the upper part o'f the profile composed of loose rocks, wells are drilled with the use of mud solutions. On casing and cementation, the lower part of the profile with solid walls is drilled and the well is flushed with clean water. When prospecting deposits of steam it is essential to have a pumping plant (with a capacity of 25-30, or more, I/see) to cool the borehole and prevent steam and water blow-outs. The temperature of the outflowing flushing liquid must not exceed 80-85 °C. On completion of drilling, screen columns with perforated tubes at the points of productive intervals are lowered into the prospecting wells. This is followed by a long-term exploratory release of thermal waters from a group of wells with maximum yields until stabilization of discharge and corresponding decline in the dynamic level are achieved. Low productive and unproductive wells serve as the observation ones. For instance, on the Pauzhetka deposit of steam, 10 prospecting boreholes were productive and 11 served as observation ones. The duration of an exploratory release of thermal waters or steam may differ (e.g., 3-5 months to one year). It is minimal on small deposits located in granite massifs and maximum on the deposits of steam. If the water levels in the observation wells do not stabilize for a long time, but, on the contrary, gradually decline under a constant yield of the water intake, this can indicate that the release of thermal water 1179
(steam and water mixture) exceeds the natural reserves, and it is necessary to decrease the water discharge by cutting off a number of wells or shutting the slidevalves. A considerable drop in water temperature and changes of water composition in the marginal wells are also indicative of the drawing up of surrounding cold waters. An exploratory release makes it possible to assess the true volume of exploitable reserves of the deposit (site) as well as stability in time of water composition and temperature. Such a release crowns prospeering works, and its results serve as the basis for the determination of the exploitable reserves of the deposit, elaboration of certain standards (quantity and quality of water or steam and water mixture, conditions of exploitation, indices of economic effectiveness) and compilation of the scheme for water intake exploitation based on the parameters determined in the course of prospecting. The geological-commercial assessment of the various deposits of thermal waters or steam presents some difficulties due to the various types of utilization (heating of dwelling houses, greenhouses, hotbeds, etc; hot water supply; in technological processes of washing, drying, fermentation, etc; in Russian baths, shower baths, laundries, etc.; in balneology; for filling swimming pools; in the extraction of chemically valuable components; for electric power generation in a number of regions of recent volcanicity). The possibility of such a multipurpose utilization of thermal waters hampers elaboration of the technical and economic indices and determination of the economic efficiency of their utilization. Complex utilization of thermal waters by a certain group of consumers is the main prerequisite for an efficient exploitation of their deposits, raising profitability and promoting a more complete use of the potential heat. Since thermal waters are regarded as a new source of thermal energy, the geological-commercial assessment of their deposits should be generally based on the above aspects. Geological - Economical water deposits
assessment
of
thermal
Up till the present moment, the deposits of thermal waters from the viewpoint of their utilization have been mainly assessed by taking into account their hydrogeologieal, therrnodynamie and power generation aspects (1). Unfortunately, investigations conducted in this field did not deal sufficiently with the problems relating to the economics of reconnoitring, prospecting and utilization of thermal water deposits, though the geological-economic analysis should be regarded as one of the main elements of geological prospecting, primarily aimed (1) This paper does not deal with the geological-eeonomical assessment of natural steam deposits intended for electric power generation. 1180
at establishing their relative significance for the people's economy and priority of commercial development. Hydrogeologists are confronted with the above problems at all stages of prospecting until the very moment of utilizing the deposit for commercial development. Underestimation of the geological-economic assessment results in a detailed prospecting of deposits which presents no value for industry now and will be useless in the future. It goes without saying that at various stages of geological prospecting the purposes of the geological-economic assessment will be quite different, as every stage pursues its own objects. At the stage of reconnoitring and prospecting it is already possible to have a sufficiently correct view of the commercial type and amount of the thermal water deposit and of its quality. Hence it is possible to employ not only hydrogeological but also economic methods of assessment. Economic assessment at this stage helps to establish the potential value of the deposit and to come to a conclusion on the expediency of prospecting. Such assessment should be based on the minimal number of technical and economic indices relating to the reserves and quality of thermal waters. The parameters to be assessed are as follows: 1)- Possible magnitude of reserves. 2) - Their quality.
3)- Conditions of occurrence and formation of subterranean thermal waters. 4) - Tentative value of the deposit in terms of money, etc. The geological-economic assessment at the stage of preliminary prospecting of a thermal water deposit should concentrate on the solution of problems relating to the exoediency of its development. The geological-economic assessment based on the data of preliminary prospecting should characterize the future raw material base. It should elucidate the natural and economic factors, which are of great importance for selecting a variant of the deposit development, and contain an analysis of the effect of these factors on the selection of the main technological solutions. On the basis of all the results obtained from the preliminary prospecting, the technical-economic substantiations or reports (TES or TER) are compiled. These reflect the economic expediency and establish the priority of commercial development of the given deposit. While compiling TES or TER, the following technical and economic indices should be taken into account:
Natural value of the deposit a) - reserves of thermal waters; b)- useful comvonents (thermal capacity, balneologieal significance, etc.); c) - value of the deposit in terms of money.
EHiciency o[ the exploitation a) b) c) d)-
possible annual capacity of wells; cost price of exploitation (with regard to extraction); profitability of exploitation; level of profitability in per cent to basic assets.
EHectiveness o[ capita investments a)- capital investments on construction of water intakes; b) - specific capital investments; c) - the term of justifying the capital investments (with regard to main objects); d) - level of profitability in relation to the invested money. Thus, the assessment indices (taking into account the purpose and objectives of preliminary prospecting) are calculated with regard to the total reserves of the deposit of thermal waters without detailing for separate blocks, sites and aquifers. In its concluding part, the TER contains data on the relative commercial value of the deposit and recommendations for the carrying out of detailed prospecting. At the stage of detailed prospecting it is significant to determine: I) - The possible consumers (depending on the properties of thermal waters and presence of useful components) which are essential for the selection of the best variant of utilization of thermal waters requiring minimum investments. 2) - Reserves of thermal waters and average capacity of wells. 3) - Amount of investments and specific capital expenditures on the deposit development. 4) . Cost price of a unit of the finished product (at the stage of extraction and consumption). 5) - Probable income to be derived, and the term of justification of the invested money. In other words, the technical and economic indices calculated at the stage of preliminary prospecting are further specified at the stage of detailed prospecting. Besides, the assessment indices should now be determined not only for the thermal water deposit as a whole, but also for separate sites (aquifers, blocks, parts of water intakes, etc.), and such indices as the cost price of the finished product, probable income to be derived, and capital investments. Their efficiency and terms of justification should be calculated at the stage of extraction and at every subsequent stage up to utilization by the consumers. Numerical expression of every assessment index is found through calculation. The initial figures should be obtained on the basis of a profound and apprehensive investigation of the particular conditions of the thermal water deposits. It should be noted that the geological-economic assessment at the stage of prospecting should be carded out in a complete way, taking into account all the useful properties of thermal waters, i.e. the criteria to be assessed are as follows:
1). Hydrogeological quantitative and qualitative characteristics of the deposit. 2)- Hydrogeological, technological and heat engineering factors: the method and system of exploitation of the deposit (spouting or pumping off), the possible capacity of the wells and the scheme of their locations, the rational " amount of wells in a water intake, the various possible uses of the thermal waters, etc. These criteria (taking into consideration the conditions of the deposit) constitute the basis for establishing the above enumerated economic indices. The geological-economic assessment carried out at the stage of detailed prospecting is very significant as it determines the future of the deposit. That is why it should be accomplished by a number of specialists, viz., hydrogeologists, economists, experts in heat engineering and power generation, agronomists, balneologists, etc. While assessing a deposit of thermal water, it is logical to consider utilization of thermal waters from the moment of extraction till the moment of supply (through pipelines) to the consumers and to make the assessment according to the finished product (1 cal of heat, 1 kWh, 1 ton of iodine, 1 ton of vegetables, etc.) and not only according to the cost of extraction of 1 m s of thermal water (the half-finished product in this case). This is very essential for planning capital investments in the industries utilizing useful properties of thermal waters. Evaluation of a deposit according to the finished product can help to assess unprofitability correctly, or economy of the thermal water intakes. The cost price of a unit of finished product reflects a number of indices such as level of investments. exploitational expenses, capacity of separate wells and heat potential. The cost price of 1 m 3 of thermal water is found from the formula: tl
C., = v_ k~"~i + E.Ce + W , . N + 0 1
where C . . . . cost price of 1 m 3 of the extracted water; ki - - c a p i t a l investments per I m 3 of water; ~i - - deduction from the basic assets (taking into account the normal coefficient 0.15); E - - e x p e n d i t u r e of electric energy on extraction of 1 m 3 of water; Ce - - c o s t
of 1 kWh of electric energy;
W p - - w a g e s of the personnel; N - - n u m b e r of workers; O - - o t h e r expenses on extraction of 1 m 3 of water amounting to 10 per cent of the exploitational expenses; Table 5 contains data on the cost price of 1 m 3 of thermal waters and 1 kcal of heat at the stage of deposit prospecting. 1181
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Determination of the cost price of 1 m 3 of thermal water and 1 kcal of heat was based on the following assumptions: 1)- When carrying out prospecting of fissure-vein or stratal thermal water deposits, the depths of wells do not exceed 300-500 and 1500-2500 (seldom more) metres respectively. 2) - Deposits of the first type are bored by column drills and those of the second type by rotary drills. 3) - The amount of the obtained heat was determined taking into account the lower temperature limit of water 200 C 0). 4 ) - The yields of individual wells (as has been proved by experience in prospecting and developing thermal water
deposits) fluctuate from 5 to 20 or more l/see, and the water temperature at the well mouth generally fluctuates from 40 to 90°C. Therefore, while calculating the cost price of 1 m 3 of water and 1 kcal of the obtained heat, we have assumed the followin~ yield values of the individual wells of a water intake Q): (a) for spouting wells: 5, 10. 15 and 20 1/sec; (b) fbr pumped wells: 10, 20, 30 1/see.
Besides, we have determined the influence of interaction of wells in a water intake (at the interaction coefficients of 0.2, 0.5, 0.7) on the cost price of water. A high coefficient of interaction is most frecluentlv observed in prospecting the fissure-vein deposits. Therefore, the changes in the cost price have been calculated for this type of deposit only. Table 5 contains all the data relating to the calculations of the cost price of extraction of 1 m 3 of thermal water and for obtaining 1 kcal of heat in the USSR's three most typical regions of thermal water occurrence: the north Caucasus, Uzbekistan and west Siberia. These data were compared with the tariffs on thermal energy in the regions in question. A thick line separates those conditions of prospecting under which the cost price of I kcal does not exceed the tariffs of thermal energy. Table 5 demonstrates that the cost price of 1 kcal of heat is below that envisaged by the tariffs for the wells 300-500 m deep, with a yield of 5 and more l/see and a water temperature equal to, exceeding, 40 °C. If a well is 1500-2000 m deep, the cost price of the obtained heat is below the tariff one at a yield of 15 1/see and water temperature exceeding 70°C. When the wells are 2500 m deep the respective figures are I0 I/sec and 80 °C. It should be pointed out that the pumping of thermal waters, while increasing the cost price of 1 m 3 of water and I kcal of heat somewhat, considerably increases the heat potential of a strata1 thermal water deposit. Moreover, a smaller number of wells may be required to provide for the same thermal loading; therefore, the possibility of deep pumping must be consid(D Geothermal heat is mainly used by consumers for heating and hot water supply. In the first case the lower temperature limit is assumed to be 40°C and in the second, 5°C. We have assumed an average temperature limit of 20°C. C-) For the calculations it was conditionally assumed that the water intake consisted of 5-8 wells.
ered when elaborating projects on exploitation of deposits. One should bear in mind that a comparison of thermal heat cost price with the tariffs of thermal energy makes the performed calculations more reliable. Table 5 contains the tariffs on heat supplied by large central heating plants or district boiler houses, whereas the geothermal heat is generally utilized in ~mall capacity plants intended for a non-centralized heat consumption. Therefore, the cost price of 1 kcal of heat produced by the latter is twice as large. The calculations prove that the cost price of a unit of produce increases by 2-3 times where the coefficient of interaction of wells changes from 0.2 to 0.7. Up till the present moment, thermal water deposits have been assessed (if at all) according to their socalled <
]~HHAeMaH H. H. 1963. OtlteHKa aKCnhyaTaI.IMOHHraIX 3al-laCOB nOA3eMHbIX BOA. FoczeovexuaOav. BnHAeMaH H. H., BoqeBep ~ . M. 1964. PerrioHaAbHa~[ oIleHKa 3KCrIAVaTauriOHHblX 3ariaCOB IIOA3eMHblX BOA. Hec)pa. Moc~coa. Boqeaep ~ . M., BepHrnH H. H. 1961. MeTOArI~eCKOe noco6rie no p a c q e T a m :~KCrIAyaTaIII,IOHHHX 3 a n o c o B HOA3eMHblX BOA AA$I BOAOCHa6>KeHH~.
Focyc)apcveennoe uac)arenz,reo nuTeparypt, t no crpourenbcvay, apxurewrype u crpourem,nbt• a~arepua.aaa. Moc~coa. BbIxoaep H. A., Ka6axn•ae A. rI., Certarrig F. II. rt Ap. 1968. :gKortoMriKa UriHepaAbnoro cr,xpb a r t reoAoro-paaaeAoqHblX pa6oT. /430. Hec)pa. FOAb6epr B. M., ~Iamm A. C. 1965. MeToAHqecKHe yKa3anna rio oUeHKe aKCnAyaTaI~HOHHbIX 3anacoB TepmaAbHblX BO,A,. I/I30. /4HCTUTt4Ta BCEFHHFEO. MaBprmKHh B. ~ . , AHTOHeHKO F. K. 1967. Onr:,lT HCCAeAOBaHH~I,
paaBeAKn
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B npaKTHqecKnx iaeA~iX TepMaAI~HI,IX BOA a CCCP I4 3a p¥6emoM. HeO.t)a. MaBpHIJKHI,~ ]~. 0:15. 1967. HeKoTopbm BorrpocI,I qbopMHpOBaHHSl TepMaAbHi,IX BOA, H IlepClIeKTrlBbl HK HClIOAb3OBaHH~I. Coeerczea.q zeonozun.
MaCKeT T. 1948. TeqeHae OAHOpOAH~,IX mHAKOCTefi B nopncTO~ cpeAe, rocronrexu3c)ar. rIAOTHHKOB H. A. 1959. OtleHKa 3anacoB nOA3eMHI~IX BOA. Foczeonvexuac)ar. IIAoTHrlKOB H. H. 1968. IIOnCKH ~I paaaeAKa npeCHI~IX IIOA3eMHIaIX BOA AA~;I IIeAefi K p y r i H o r o
BOAOCna6x~enrlst. ~t. FI. uab. MFY. 1967. PerrtoHahi, Ha~ reoTepMi~a H pacrlpocTpaHenrle TepMaAI~HI,IX BOA B CCCP. TO. a o o p o r o coBem.iIO reoTepMrtqecKriM ~ICCAeAOBaHIa~M B CCCP. H30. Hav~ca, M. II.l~eAKaqeB B. hi. 1959. x]'nlaVr~Ifi pe>KnM rtAac'rOBblX BOAOHarlOpHlalX CHCTeM. FOCTOnTexu3c)aT.
1183